The robotic explorer Juno is set to become the most distant probe ever powered by the sun.

Juno is equipped with three tractor-trailer-size solar panels for its 2 billion-mile (3.2 billion kilometer) journey into the outer solar system. It's due to be launched at 11:34 a.m. ET Friday aboard an unmanned Atlas 5 rocket — barely two weeks after NASA's final space shuttle flight.

The shuttle's demise is giving extra oomph to the $1.1 billion voyage to the largest and probably oldest planet in the solar system. It's the first of three high-profile astronomy missions coming up for NASA in the next four months.

Jupiter — a planet several NASA spacecraft have studied before — is so vast it could hold everything else in the solar system, minus the sun. Scientists hope to learn more about planetary origins through Juno's exploration of the giant gas-filled planet, a body far different from rocky Earth and Mars.

"Look at it this way — it is a new era," said Jim Green, NASA's director of planetary science. "Humans plan to go beyond low-Earth orbit. When we do that, it's not like 'Star Trek.' It's not 'go where no man has gone before.'"

Southwest Research Institute astrophysicist Scott Bolton, Juno's principal investigator, said it's also important for people to realize "NASA's not going out of business."

"If we're going to learn who we are and where we came from, and how the earth works, we've got to keep doing these science missions, not just Juno," Bolton said.

NASA's long-range blueprint would have astronauts reach an asteroid by 2025 and Earth's next-door neighbor Mars a decade later, although there's still uncertainty surrounding the rockets needed for the job. A Juno success would be a good sign for future solar-powered missions of all types.

Jupiter may be just two planets over, but it's far enough away to be considered part of the outer solar system.

It will take Juno five years to reach its target, five times farther from the sun than Earth. No spacecraft has ever ventured so far powered exclusively by solar wings. Europe's solar-powered, comet-chasing Rosetta probe made it as far as the asteroid belt between the orbits of Mars and Jupiter.

AP

Technicians at Astrotech's payload processing facility in Titusville, Fla., stow one of Juno's solar arrays against the spacecraft's body in preparation for flight. The robotic explorer Juno is set to become the most distant probe powered completely by the sun.

Each of Juno's three wings is 29 feet (8.8 meters) long and 9 feet (2.7 meters) wide, necessary given that Jupiter receives 25 times less sunlight than Earth. The panels — folded for launch — emanate from the spacecraft much like the blades of a windmill.

At Jupiter, nearly 500 million miles (800 million kilometers) from the sun, Juno's panels will provide 400 watts of power. In orbit around Earth, these panels would generate 35 times as much power.

The choice of solar was a practical one, Bolton said. No plutonium-powered generators were available to him and his San Antonio-based team nearly a decade ago, so they opted for solar panels rather than develop a new nuclear source. They wanted to avoid ballooning costs and possible delays connected with developing new technologies.

"It's nice to be green, but it wasn't because we were afraid of the plutonium," Bolton explained.

Indeed, NASA's six-wheeled, Jeep-size Mars rover named Curiosity, due to launch in late November, will be powered by more than 10 pounds (4.5 kilograms) of plutonium. Despite safety efforts, there's always the question of public safety if an explosion occurred.

Eight robotic craft already have flown to or near Jupiter and its many moons, as far back as the 1970s: NASA's Voyagers and Pioneers, Galileo, Ulysses, Cassini and, most recently in 2007, the Pluto-bound New Horizons.

Juno — named after the cloud-piercing wife of Jupiter, the Roman god — will go into an oval-shaped orbit around Jupiter's poles in July 2016, after traveling 1.74 billion miles (2.8 billion kilometers).

The craft will fly within 3,100 miles (5,000 kilometers) of the dense cloud tops, closer than any previous spacecraft. Any closer, and Juno would feel the tug of the planet's atmosphere, which in turn would alter the spacecraft's orbiting path and hamper its gravity experiment.

The spinning spacecraft will circle the planet for at least a year, beaming back data that should help explain the composition of its mysterious insides. Each orbit will last 11 days, for a total of 33 orbits covering 348 million miles (560 million kilometers).

Nine instruments are on board, including JunoCam, a wide-angle color camera that will beam back images.

Science editor Alan Boyle's blog: "Astronaut Abby" is at the controls of a social-media machine that is launching the 15-year-old from Minnesota to Kazakhstan this month for the liftoff of the International Space Station's next crew.

Juno's most sensitive electronics are inside a titanium vault to protect against the incredibly harsh radiation surrounding the planet. The radiation exposure will worsen toward the end of the mission. "We're basically an armored tank going to Jupiter," Bolton said.

Scientists believe Jupiter was formed from most of the leftovers of the sun's creation. That's why it's so intriguing; by identifying the planet's contents, besides hydrogen and helium, astronomers can better explain how the solar system came to be.

"We want to know that ingredient list" for Jupiter, Bolton said. "What we're really after is discovering the recipe for making planets."

For these answers, Juno will study Jupiter's gravity and magnetic fields, and turbulent, cloud-socked atmosphere, which can spawn 300 mph (480 kph) wind and hurricanes double the size of Earth. The experiments will investigate the abundance of water, and oxygen, in Jupiter's atmosphere and help determine whether the planet's core is solid or gaseous.

Once its work is done in 2017, Juno will make a kamikaze dive into Jupiter. NASA doesn't want the spacecraft hanging around and crashing into Europa or other moons, possibly contaminating them for future generations of explorers.

Jupiter loses a stripe

The weather on Jupiter is changeable, as these before-and-after pictures show. The photograph on the left shows Jupiter as seen in June 2009. The photo on the right, taken on May 9, 2010, reveals that one of the planet's prominent dark cloud belts has faded away. The lightening of the South Equatorial Belt is due to atmospheric changes. Both pictures were taken by Anthony Wesley, an amateur astronomer in Australia.
(Anthony Wesley via The Planetary Society)
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Family portrait

Launched in 1989, the Galileo spacecraft has photographed Jupiter as well as several of the giant planet's satellites. Here's a montage that shows Jupiter's Great Red Spot and the four largest moons. From top, they are Io, Europa, Ganymede and Callisto.
(NASA)
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Cratered Callisto

Callisto is considered the most cratered celestial body in the solar system. The false-color overlay at right exaggerates the moon's surface features, including the Valhalla impact structure near the center of the disk.
(NASA)
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Dark face

Colors are enhanced in this view of Ganymede's trailing hemisphere, highlighting the moon's polar caps. The violet color indicates where small particles of frost may be scattering light on the blue end of the spectrum.
(NASA)
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Cloudy weather

The mosaic at left shows the true colors of the cloud patterns in Jupiter's northern hemisphere. The rendition at right uses false colors to represent the height and thickness of the cloud cover.
(NASA)
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This is the Spot

A true-color picture captures the subtle shadings of Jupiter's Great Red Spot, a massive, long-lived storm system in the planet's thick atmosphere.
(NASA)
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A big splash on Europa

A computer-generated perspective view shows the Pwyll impact crater on Europa, an ice-covered moon of Jupiter. The heights are exaggerated, but the central peak indicates that the crater may have been modified shortly after its formation by the flow of underlying warm ice.
(NASA)
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A blast at Io

This image of Io, thought to be the solar system's most volcanically active world, shows the plumes of two eruptions. One plume can be seen at the very edge of the disk, the other is puffing up from the dark volcanic ring near the center of the disk.
(NASA)
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Lava light

An active volcanic eruption on Jupiter's moon Io flares in an image taken in February 2000 by the Galileo spacecraft. The dark L-shaped lava flow to the left of center marks the site of energetic eruptions in November 1999 at Tvashtar Catena, which is a chain of giant volcanic calderas. The two small bright spots at left side of image are sites where molten rock is exposed to the surface at the toes of lava flows.
(NASA)
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Crazy quilt

The thin crust of Europa's Conamara region is criss-crossed by craters, cracks and lines - indicating that the surface ice was repeatedly disrupted. The colors, which are enhanced in this view, show where light ice crystals and dark contaminants have settled onto the surface.
(NASA)
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A moving moon

In a picture taken in April 2001 by NASA's Cassini spacecraft, the moon Io looks like a marble set against the background of Jupiter. Io is the giant planet's third-largest satellite.
(NASA)
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